An offshore subsea well normally is drilled to a first depth, then an outer or low pressure wellhead housing with a string of conductor pipe or outer casing is installed. The operator then will normally drill the well to a second depth and install an inner string of casing within the conductor pipe. An inner or high pressure wellhead housing is secured to the upper end of the inner string of casing and landed in the low pressure wellhead housing. A drilling riser with a blowout preventer is attached to the high pressure wellhead housing. The operator then drills the well deeper through the drilling riser and runs one or more additional strings of inner casing.
Offshore well drilling is normally performed with a drill bit attached to drill pipe. After drilling each section to a desired depth, the operator runs and cements a string of casing in the well. After retrieving the drill pipe and before running the casing, the wellbore is prevented from collapsing by the mudcake on the borehole wall and the weight of the drilling mud in the borehole. In certain geographic zones, the open borehole tends to collapse easily because of they may be unconsolidated, and formation pressure causes water flow from the formation into the borehole.
One known technique for well drilling is called “casing-while-drilling”. In that technique, the drill bit is secured to the lower end of a string of casing and lowered into the wellbore. The bit is rotated either by rotating the casing or by pumping drilling fluid to a mud motor attached to the casing. After the desired depth is reached, the casing is cemented. The drill bit may be an expendable type that remains cemented in the wellbore. Alternately, it might be retrieved through the casing before cementing. If a mud motor is employed, it normally would be retrieved along with the bit before cementing the well. Casing-while-drilling has a number of advantages. One of them would be to prevent the borehole in certain formations from collapsing prior to cementing.
The currently available equipment for casing-while-drilling is designed for use in land wells. A land well normally would not have a low pressure wellhead housing and a high pressure wellhead housing. One problem associated with using casing-while-drilling for drilling with the subsea outer string of casing is that the outer string of casing is typically smaller in inner diameter than the high pressure wellhead housing to which it is attached. Common dimensions are 18¾ inches internal diameter for the high pressure wellhead housing and 20 inches internal diameter for the first inner string of casing. A retrievable drill bit, either with or without a mud motor, for drilling the hole for the 20 inch casing would normally be large to pull through the 18¾ inch high pressure wellhead housing. Using casing-while-drilling to install the 20 inch casing might have benefits, but is not currently being done.